The sol-gel process is a wet-chemical technique that is widely used in the fields of materials science and ceramic engineering. The process is primarily for the fabrication of materials, such as metal oxides, starting from a colloidal solution (sol) that acts as a precursor for an integrated network (or gel) of either discrete particles or network polymers. The “sol” or solution gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks.
Aerogel is a synthetic, porous, ultralight gel with extremely low density and thermal conductivity, formed via the sol-gel process. Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This allows the liquid to be slowly dried off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation. Silica aerogel is the most common type of aerogel and the most extensively studied and used.
A xerogel is another gel formed via the sol-gel process, however, instead of being dried by supercritical drying, xerogels are dried by evaporation. Xerogels usually retain high porosity and enormous surface area along with very small pore size. The main distinction between aerogel and xerogel rests on their densities. Aerogel is typically 90-99% air while xerogel is typically 60-99% air.